1. Field of the Invention
The present invention relates to a light emitting element display device.
2. Description of the Related Art
In recent years, light emitting element display devices using self-luminous bodies such as organic light emitting diodes (OLED) have been put into practical use. As compared with the related art liquid crystal display device, because the light emitting element display devices including an organic EL (Electroluminescent) display device using the OLED use the self-luminous bodies, the light emitting element display device not only is excellent in visibility and response speed, but also requires no auxiliary lighting device such as backlight, and therefore can be further thinned.
JP 2002-352955 A discloses a display device having the light emitting elements in which a drive element is formed below a bank, and a lower light shielding film having conductivity is formed below the drive element, to thereby prevent a leakage current, and form a bank layer by self-alignment.
In the organic EL display device, in recent years, high definition progresses, to thereby reduce the size of each pixel. The organic EL display device allows a current to flow by retaining a potential difference corresponding to a gradation value in each pixel. However, a reduction in the size of pixels causes electrodes to be also reduced in size, as a result of which a capacity for retaining the potential difference is reduced. If the capacity for retaining the potential difference is small, the potential difference changes due to an influence of noise or an influence of slight leakage, and brightness of the respective pixels is varied. As a result, it is conceivable that the display quality is degraded.
Recently, a demand for high definition and lower power consumption has been strongly required in a light emitting display device for mobile usage. As a display device for mobile usage, for example, a liquid crystal display device (LCD), a display device that uses a self-light emitting element (an organic light emitting diode (OLED)) such as an organic EL display device, and electronic paper are employed.
Among these, for the purpose of realizing a flat-screen and high brightness, and high speed of a display panel, the organic EL display device has been developed. The organic EL display device is a display device which is provided with a pixel formed of the OLED, and has no mechanical operation and thus has fast reaction rate. In addition, in the organic EL display device, light is emitted from each pixel, and the back light source is not necessary, and thus it is possible to perform high brightness display, and to realize the flat-screen. For this reason, the organic EL display device is expected as a next generation display device in place of the liquid crystal.
Particularly, in a small and medium-sized organic EL display device, high definition of a display unit has been required. When the high definition of the display device progresses, a pixel size is reduced, and thus it is necessary to reduce the sizes of a plurality of TFTs which form a pixel circuit, or a capacitor, that is, the size of a capacitor for supplying a current which corresponds to the size of a video signal to an OLED, a thin film transistor (TFT) for writing the video signal into the capacitor, a TFT for supplying the current which corresponds to an electric charge of the capacitor to the OLED from a power line, or the like. In the TFT which writes the video signal into the capacitor, there is a problem in that the variation of the video signal is increased due to leakage currents, unnecessary charges, and the like when retaining the video signal and thus the display unevenness occurs. In addition, the TFT for supplying the current to the OLED is required to have large driving performance; however, an area size is reduced. Further, it is required to reduce characteristic variation due to the application of the current.
In this regards, JP 2013-54161 A discloses a technique relating to a driving method of suppressing display unevenness caused by potential setting timing. However, in a full high definition smart phone, and the like, when high definition progresses through a control method disclosed in JP 2013-54161 A, characteristics of a transistor which writes a video signal into a capacitor are varied, thereby adversely affecting the image quality. In addition, the driving performance of the TFT is not sufficient to supply a current to an OLED.
Further, for example, JP 2012-160679 A discloses a technique relating to a display device that uses a thin film transistor of which a channel is formed on an oxide semiconductor layer. However, in a case where the TFT of which the channel is formed on the oxide semiconductor layer is used as a driving element for supplying a current to an OLED, there is a problem in that an operating point is greatly varied over time.
In addition, US 2015/0055051 A, US 2015/0053935 A, and US 2010/0182223 A disclose examples that in the organic EL display device, in a driving transistor for controlling a current which flows into an organic light emitting element, an oxide semiconductor layer is used as a channel, and in a switch transistor for controlling whether or not a gradation voltage is applied with respect to a capacitance for retaining a gate voltage of a driving transistor, a polycrystalline semiconductor layer is used as a channel. However, a leakage current amount of the transistor in which the polycrystalline semiconductor layer is used as the channel is larger than that of the transistor in which the oxide semiconductor layer is used as the channel, and thus the charge may leak from the capacitance for retaining the gate voltage of the driving transistor. Further, the current greater than that in the switch transistor flows into the driving transistor, but the transistor in which the oxide semiconductor layer is used as the channel has mobility lower than that of the transistor in which the polycrystalline semiconductor layer is used as the channel, and is required to have an area larger than that of the transistor in which the polycrystalline semiconductor layer is used as the channel when the same amount of currents flows at the same voltage between the gate sources.